- Email: [email protected]
Miniature ion mass analyzer
ARTICLE IN PRESS
Planetary and Space Science 55 (2007) 1190–1196 www.elsevier.com/locate/pss
Miniature ion mass analyzer D. McCanna, , S. Barabasha, H. Nilssona, A. Bhardwajb a
b
Swedish Institute of Space Physics, Kiruna, Sweden Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India Accepted 4 July 2006 Available online 12 February 2007
Abstract The Swedish Institute of Space Physics is developing a miniature plasma analyzer for planetary missions (MIPA—Miniature ion precipitation analyzer). MIPA has been accepted to fly on-board both the ESA BepiColombo mission to Mercury (2014) and the Indian Chandrayaan—1 mission to the Moon (2007). The analyzer measures ions in the energy range 10 eV–15 keV and has a sufficient mass resolution to resolve the main groups of ions, namely M=q ¼ 1; 2; 4; 8; 16; 430. Field of view is 9 180 . The instrument consists of the sensor head and a separate electronic board whose total mass is 300 g. The sensor head envelope is roughly 53 85 30 mm3 in volume and is designed for the extreme operation temperature range of 100 to þ125 C. MIPA comprises an electrostatic scanner for angular resolution, a cylindrical electrostatic analyzer for energy discrimination and a time-of-flight (TOF) section for particle velocity measurement. Generic design allows using the instrument on various platforms including nano-satellite and multi-spacecraft missions. This document describes the design of the sensor part including ion optical as well as mechanical aspects. r 2007 Elsevier Ltd. All rights reserved.
1. Introduction The instrument currently being developed at The Swedish Institute of Space Physics (IRF, Kiruna, Sweden) presented here has the working name MIPA (miniature ion precipitation analyzer). In MIPA’s original design it was developed for the European Space Agency BepiColombo (B/C) mission to Mercury, where it is to monitor the precipitating ions (primary) and study sputtered ions from the surface (secondary). MIPA has been proposed and accepted to be included in the SERENA (Search for Exospheric Refilling and Emitted Natural Abundances) package on-board BepiColombo MPO (Mercury Planetary Orbiter). Alongside designing for B/C, MIPA has also been proposed for, and accepted to fly on-board the Indian Chandrayaan—1 mission to the Moon. The working name for the instrument on Chandrayaan—1 is SWIM (Solar WInd Monitor). Small changes to the design (e.g. adding electronic board housing) had to be implemented to meet the constraints, but no sensor-specific properties have been altered, i.e. from a scientific point of view it is the same Corresponding author. Tel.: +46 980 79176; fax: +46 980 79050.
E-mail address: [email protected] (D. McCann). 0032-0633/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2006.11.020
instrument flying on-board both missions. See Table 1 for basic instrument characteristics. Regarding Mercury, placement on the MPO will allow MIPA to study the particles along the magnetics field lines (Sarantos et al., 2001), precipitating upon the surface and inducing sputtering (Lammer and Bauer, 1997). This is a very important component of the ENA (Energetic Neutral Atoms) package of MPO. In order to fully understand the behavior of sputtered ENA’s, one should simultaneously study the source of sputtering. Because of the small size of the Mercury magnetosphere (Kabin et al., 2000), it is only from the MPO platform one can get a good coverage of the plasma populations within the non-tail magnetosphere (Nilsson et al., 2002). Besides the primary objective of monitoring of the solar wind, and supporting the ENA studies, there are several scientific topics related to ions around the Moon that SWIM will investigate, e.g. the lunar-originated ions reflected from the surface, that could give important data about the lunar surface composition (Elphic et al., 1998). Directly connected with the ENA investigations of the possible occurrence of mini-magnetospheres (Harnett and Winglee, 2002), it would also measure ion composition in and around these. This could give information regarding
ARTICLE IN PRESS D. McCann et al. / Planetary and Space Science 55 (2007) 1190–1196 Table 1 Basic instrument characteristics
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Table 2 Resource budget summary
Parameter
Value
Resource
Energy range Energy resolution, DE=E Field-of-view Mass resolution Geometric factor
10 eV–15 keV 7% 9 180 H, He, Na/Al, K/Ca, Mn/Fe 0:14 cm2 sr eV
Mass (incl. margins) MIPA Sensor head FEE þ HVPS SWIM Sensor head FEE þ HVPS Electronic board housing
150 g 200 g 100 g
Power consumption MIPA/SWIM
1.5 W
TM MIPA SWIM
TBD 2 kbps (average)
the micro-scale interaction between the anomalies and the solar wind, as well as studying the existence (if any) of ion void regions around the anomalies. Also studying the electromagnetic environment around the terminator should be noted. The terminator is the boundary for photo-electron production, which causes a complicated environment, which in turn could perturb the solar wind (especially the solar wind protons) in this region. Studying the disturbed solar wind in this region could make it possible to map the potential structures around the terminator (Futaana, 2006).
Value
150 g 200 g
2. Mechanical design of the instrument The basic ideas for MIPA were adopted from the proposed PAEX-sensor (Pluto Atmosphere Escape Experiment) (Barabash et al., 2000). The asymmetrical detector geometry of PAEX was replaced with symmetrical plates and the pulse height analysis-based detection of PAEX was replaced by a time-of-flight system (TOF) in MIPA. Table 2 shows mass and power consumption for the instrument. A 3D drawing of the sensor interior can be seen in Fig. 1. The sensor comprises three sub-units. The first is the electrostatic deflector (Section 1 in Fig. 1) consisting of two high voltage plates, which will provide angular resolution. The second part is the electrostatic analyzer (ESA, Section 2 in Fig. 1), which will provide energy (or rather E/q) discrimination. The final sub-unit is the TOF system (Section 3 in Fig. 1), measuring the particle velocity. The sensor will make use of five high voltage supplies (see Table 3). One for each of the two deflector plates (4) and one for the analyzer plate (5). The TOF cell (6, see Section 2.3 for description) will, besides creating a field-free region for optimizing the particle trajectories, provide pre-acceleration as well as bias to the START (7) and STOP (8) surfaces. Finally, both detectors, Ceramic Channel Electron Multipliers (CCEM, 9 and 10) will operate under the same high voltage supply. 2.1. Electrostatic deflector
The deflector consists of two 90 curved high voltage plates with a radius of 19 mm, separated by 2 mm at the aperture. The entrance aperture is entirely covered with a grounded grid (not shown in Fig. 1), which is transparent for ions (transparency efficiency is 90%), so no electric
Fig. 1. 3D view of the sensor head with pointers to the significant subunits and details. The sensor comprise three sub-units; (1) the electrostatic deflector, (2) the electrostatic analyzer and (3) the time-of-flight system. Also seen is the deflector high-voltage plates (4), the analyzer high-voltage plates (5), the time-of-flight cell (6), the START/conversion plate (7), the STOP plate (8), the START and STOP ceramic channel electron multipliers (9, 10) and the UV attenuation block (11).
Table 3 Sensor high voltage supplies Supplies
Interval (kV)
Comment
Upper defl. Lower defl. Analyzer CCEM TOF cell
½3; 5; 0 ½3; 5; 0 ½3; 5; 0 ½0; 5; 0 ½1; 0; 0
Sweeping Sweeping Sweeping Constant Constant
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D. McCann et al. / Planetary and Space Science 55 (2007) 1190–1196
field will be visible from outside the sensor head. Each of the two deflector plates will scan 90 of the azimuthal acceptance angle. The plate not scanning will be held at ground potential, while scanning will be performed in the interval 0–3.5 kV. In each part of the 180 field-of-view scanning will be performed in four steps for each energy level. Including the zero position (both deflector plates at ground potential) this will provide nine azimuthal sectors. Some problems have been encountered when looking in the extreme angle (90 direction). The angular acceptance distribution becomes asymmetrical (in Section 4 we address this issue in detail). 2.2. Electrostatic analyzer The electrostatic deflector is separated from the electrostatic analyzer by a grounded slit, as is the analyzer from the TOF module. The width of the electrostatic analyzer channel is 2 mm and the height is 20 mm, whereas the entrance- and exit slits have dimensions of 1:5 8:0 mm. This decreases the distorting impact on the fringing fields from the top and bottom edges of the analyzer channel will have on the particle trajectories. The analyzer has a radius of 19 mm and a curvature of 127 , providing focusing of the particle trajectories at the ESA exit (Hughes and Rojansky, 1929). Since, as the case for the deflectors, the ESA is not based on a positive and a negative potential, it is asymmetric. The outer ESA wall is grounded, while the inner wall will be biased with a negative potential to bend the ion trajectories. Assuming a perfect logarithmical distribution for the electric field of the analyzer, it is possible to make a relation between analyzer voltage U A , ion’s kinetic energy E k and charge q for ions with circular orbit which can pass the analyzer (Johnstone, 1972): Ek UA ¼ , q 2 lnðrout =rin Þ
(1)
where the analyzer constant (1=ð2 ln rout =rin Þ) is roughly 5 for MIPA. 2.3. Time-of-flight section Pre-acceleration of 1 keV is used in order to enhance the electron yield from the conversion surface for low-energy ions. In order to suppress electric fields disturbing the particle trajectories in the TOF section, we implemented a structure named the TOF cell. All but for the windows where the two CCEM’s collection plates are placed, it completely closes in the TOF section, creating as field-free region as possible. Fig. 2 shows a zoomed-in view of the TOF cell. The cell is machined out of one piece of graphite, giving uv attenuation as well as low secondary electron emittance. Particles entering the TOF cell will first hit the START surface, giving rise to secondary electron yield, which will be detected by the START CCEM. The ions reflecting off
Fig. 2. Zoomed-in view of the time-of-flight cell. A potential of 1 keV will be applied on the cell, which is machined out of a solid piece of graphite. Graphite will provide both photon flux attenuation, without blackening process, and electron absorption (cmpr e.g. aluminum).
of the START surface will scatter towards the STOP surface, again yielding secondary electrons, which here will be detected by another (STOP) CCEM. Timing of the two detections gives the TOF and since we know the average distance the particle has to travel between the start and stop surface we can determine the velocity. Average TOF distance is 3.1 cm, which corresponds to TOF in the interval a few tens of ns to a couple of ms. CCEMs of type KBL-408 from Dr. Sjuts Optotechnik GmbH were selected for the TOF-system. These will be run typically with an internal potential drop of 2.3 kV. In order to compensate for aging the potential drop can be raised to some 3.3–3.5 kV with an increasing gain. The START and STOP surfaces will be biased to negative high voltage ð1 kVÞ in order to increase the production of secondary electrons as well as guiding the yielded secondary electrons towards the CCEMs. 2.4. UV attenuation Both for the BepiColombo and Chandrayaan—1 mission the instrument aperture will at times be in direct lineof-sight of the sun. In order to suppress ultraviolet radiation at the detector inputs, a photon attenuation structure is added right after the entrance slit of the analyzer. The basic idea of the ‘‘photon trap’’ has been adopted from ASPERA ELS instrument’s top-hat analyzer (Slater, 2000). The trap is a stack of thin copper plates which form a baffle structure having openings of 0.2 mm for photons to fly in. The plates are 0.1 mm thick and are straight cut which will make them act as mirrors for photons hitting the sides of the plates and bounce them back towards the entrance aperture. The length of the plates will follow the ESA’s outer wall radius. Fig. 3 shows a zoomed-in view of the photon trap. The trap as well as all other surfaces in the ion optics of the instrument will be
ARTICLE IN PRESS D. McCann et al. / Planetary and Space Science 55 (2007) 1190–1196
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Kiruna. The baseline for the TOF electronics can be studied in detail in Svensson and Karlsson, 2002. TOFtiming is performed using A101 charge sensitive preamplifiers (see Fig. 4) for schematic) with a rise time of about 6 ns. A prototype (see Fig. 5) of the TOF electronics has been manufactured and tested for functionality. The HVPS uses analog inputs of 12 V and þ5 V to generate five high voltage outputs. The output high voltages are shown in Table 3. Fig. 6 shows a prototype of a fully functional high-voltage supply for SWIM. 4. Simulations and tests
Fig. 3. Zoomed view of UV attenuation structure. The black body is the analyzer high voltage plate. Photons entering the analyzer channel will either propagate in between the plates and be absorbed or bounce back off of the straight edges. Simulations have indicated as high attenuation as 1010 , but mechanical/integration aspects of the very thin plates makes simulations a bit off-hand.
SimIon 7.0 and a non-commercial particle ray-tracing software (named Trace) have been used for simulations. Fig. 7 shows that for angles up to 60 the angular acceptance is, at least piecewise, symmetric. Again, due to asymmetric geometry the angular acceptance will be
Fig. 4. Simple schematic of the implementation of the A101 charge sensitive amplifiers.
Fig. 6. A picture of a fully functional technological model prototype of the high voltage power supply for SWIM. Board envelope is 120 60 mm.
Anglular distribution 5000
4000
3000
Fig. 5. A picture of a fully functional technological model prototype of the time-of-flight electronics designed and manufactured at IRF, Kiruna. Board envelope is 120 60 mm.
blackened with Ebanol-C treatment in order to reduce reflections.
2000
1000
0 –80
3. Time-of-flight electronics and high voltage power supply Both the TOF electronics and the high voltage power supply (HVPS) are designed and manufactured at IRF,
–60
–40
–20 0 Angle [degrees]
20
40
60
Fig. 7. Plot showing the angular distributions for the interval ½70; 60 . The small asymmetric behavior for positive/negative values is due to geometric factors and cannot be avoided.
ARTICLE IN PRESS D. McCann et al. / Planetary and Space Science 55 (2007) 1190–1196
different for particles coming from the, by convention, negative angles, since they will not have the same smooth orbital trajectory, as particles incident from positive angles. As seen in the figure, the angular resolution will be a bit more crude for negative acceptance angles. Fig. 8 shows trajectories from START plate to START CCEM for secondary electrons yielded after particles hit the surface. By applying potentials both on the conversion surface (negative) and the detector input (positive) the particles can be guided towards the detectors. Detection efficiency on START plate is X95%. The efficiency of detecting STOP signals is a bit lower (approximately 70%). It is also more dependent on the cell potential. This is due to the obvious reason that the average distance ðdÞ the secondary electrons have to travel is larger than for the electrons yielded at the START surface, but also that the Dd is much larger (i.e. the difference between the shortest TOF for the yielded electrons and the longest). Fig. 9 shows trajectories for secondary electrons yielded at the STOP surface being detected at STOP CCEM. The detection efficiency is quite a bit lower than for electrons yielded at START surface (70%), but as seen in the figure electrons from the entire width of the surface is detected. The detection loss is impinged for particle spread in z (vertical) direction. 4.1. CCEM thermal tests
Fig. 9. Particle trajectories from simulations for second electrons yielded at the STOP surface being detected at STOP CCEM. The detection efficiency is quite a bit lower than for electrons yielded at START surface ð70%Þ, but as seen in the figure electrons from the entire width of the surface is detected. The detection loss is impinged for particle spread in z (vertical) direction.
Normalised gain
1194
1
0.5
0
Another issue, which is valid for the BepiColombo mission, is that the instrument will be exposed to a significant heating. Thermal calculations have shown that the aperture can be heated as high as almost 300 C. This will not have implications for the electronics, as the electronics board housing will be inside the S/C wall, but it means that the CCEMs will operate under high
Fig. 8. START CCEM secondary electron collection from ray-tracing simulations. Detection efficiency of electrons from the START surface quite is high ðX95%Þ.
35
45
55
65
75 T [C]
85
95
105
115
Fig. 10. Plot showing the normalized degradation in gain as a function of temperature of the CCEM’s. Full gain corresponds to 108 . This test was just instantaneous gain fall due to temperature increases, not long time duration.
temperature decreasing the signal-to-noise ratio. The temperature inside the instrument will not be as high as at the aperture, but we can anticipate a temperature of some 125 C. Thermal-vacuum tests have been made to investigate the effect of high temperature operations of the CCEMs. Fig. 10 shows the normalized gain drop as a function of temperature. The CCEM bias is held constant throughout the test. Studies of how the noise would decrease with time (accumulated counts) are very important, since, at high temperatures, the noise could be a significant source of error. Fig. 11 shows how the gain (top) and noise (bottom) drop with accumulated counts on the CCEM. The temperature was held constant at 125 C throughout the test period and the CCEM bias was held constant at 2.3 keV. The gain stabilizes at about an order of magnitude attenuation, which still gives an acceptable gain. The noise stabilizes at some 30 counts/s, which gives a correlated noise of about 1 counts/s. These noise levels are fully acceptable.
ARTICLE IN PRESS D. McCann et al. / Planetary and Space Science 55 (2007) 1190–1196 1.20
SWIM TM Time-Of-Flight Spectra 3keV H2O+ 10
0.80 0.60 0.40 0.20 0.00 8.64E+07
5.18E+08
9.50E+08 1.38E+09 Accumulated counts
1.81E+09
300
Counts per second per ns tof time [cps/ns]
Normalised gain
1.00
H2O+
Noise [Hz]
100
0 8.64E+07
5.18E+08
9.50E+08 1.38E+09 Accumulated counts
1.81E+09
Fig. 11. Top: the normalized gain drop due to degradation of the CCEM at high temperature. The gain stabilizes at about an order of magnitude attenuation, which still gives an acceptable gain. Bottom: simultaneous noise drop with accumulated counts. Noise of the order of tens of Hz will not be a source of error. The temperature was held constant at 125 C and the potential drop at nominal 2.3 kV.
SWIM TM Time-Of-Flight Spectra 1.6 +
H2 2.1keV H2O+ 2.1keV
1.4 H+ recoil
Normalized counts
1.2
raw data 2.1keV H2O fit: peak 252ns raw data 2.1keV H2 fit: peak 81ns fastest H recoil @ 2.1keV
1 0.8 0.6 0.4 0.2
0
100
200
300
400
TOF [ns] þ Fig. 12. Two spectra overlaid: 2.1 keV Hþ 2 and 2.1 keV H2 0 . The heavy projectiles (water) show a recoil peak, whereas in the Hþ 2 case, no separated peak is visible due to closer mass match. Positions of the individual peaks are as expected. The dotted line represents the fastest possible flight time for recoil Hþ .
5. Calibrations Initial calibrations have begun for a technological model of the instrument. Calibrations is performed at IRF’s
H2O+, 3keV, Vtof = 0V H2O+, 3keV, Vtof = 500V peak: 179ns, gauss:19ns, exp: 69ns peak: 171ns, gauss:17ns, exp: 66ns
recoil H+
1 primary beam
0.1 200
0
1195
0
50
100
150
200 TOF [ns]
250
300
350
400
Fig. 13. The effect of a changed TOF cell potential as well as the recoil and nominal particles separated. The max spread in TOF introduced by the extended start surface is shown as rectangular function. The translation of the peak for a TOF cell potential (pre-acceleration) a0 is as expected.
vacuum chamber facilities in Kiruna, Sweden, and some first calibration results will follow here. Fig. 12 shows TOF distributions for two particle species, þ Hþ 2 and H2 O , at 2.1 keV. The curve for the heavy particles clearly shows a second peak in the spectrum. This peak originates from proton recoils generated the START surface. The heavy particles simply knock protons from the START surface that will reach the STOP surface before the primary heavy particle, and hence the TOF for that signal will be shorter, resulting in the second peak. This problem is partly related to the environment in the vacuum chamber and will not be as significant in space, but it will most likely not disappear entirely. The light particles ðHþ 2Þ shows a narrow single peak curve, which is as expected. Fig. 13 shows the pre-acceleration effect from increased TOF cell potential (the right-most curve is under no preacceleration, while the left-most is pre-accelerated 500 V). For the energy shown (3 keV) it will not have a significant narrowing of the peaks as for lower energies, but one can see the shift in TOFs for the different conditions. Recoil protons are clearly visible in this plot as well. Thorough investigations into the characteristics of MIPA/SWIM will be performed over the next few months, where we will look at the entire energy spectrum for the þ þ þ þ masses Hþ , Hþ 2 , H3 , N2 , H2 O and Ar . Acknowledgments Martin Wieser has contributed very much with his experience and help during the calibrations of SWIM TM. We would also like to acknowledge Peter Wurz and his team (University of Bern) for valuable assistance and help regarding certain hardware issues.
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References Barabash, S., Gimholt, J., Lundin, R., Fedorov, A., Budnik, L., Sauvaud, J-A., 2000. Pluto Atmoshpere Escape Experiment (PAEX), Swedish Institute of Space Physics. Elphic, R.C., et al., 1998. Lunar Fe and Ti abundances: comparison of lunar prospector and clementine data. Science 281 (5382), 1493–1496. Futaana, Y., private communication, 2006. Harnett, E.M., Winglee, R.M., 2002. 2.5D Particle and MHD simulations of mini-magnetospheres at the Moon. J. Geophys. Res. 107 (A12), 1421. Hughes, A.L., Rojansky, G.V., 1929. On the analysis of electronic velocities by electrostatic means. Phys. Rev. 34, 284. Johnstone, A.D., 1972. The geometric factor of a cylindrical plate electrostatic analyzer. Rev. Sci. Instrum. 43 (7).
Kabin, K., et al., 2000. Interaction of Mercury with the solar wind. Icarus 143, 397–406. Lammer, H., Bauer, S.J., 1997. Mercury’s exosphere: origin of surface sputtering and implications. Planet. Space Sci. 45 (1), 73–79. Nilsson, H., Holmstro¨m, M., Barabash, S., 2002. Feasability of a charged particle detector on the Mercury Planetary Orbiter. Internal report, Swedish Institute of Space Physics. Sarantos, H., Reiff, P.H., Hill, W.T., Killen, R.M., Urqhart, A.L., 2001. A Bx -interconnected magnetosphere model for Mercury. Planet. Space Sci. 49 (14–15), 1629–1635. Slater, D.C., 2000. ASPERA ELS UV supression study. Final report, Southwest Research Institute. Svensson, J., Karlsson, S., 2002. Time-of-flight electronics based on TDCGP1. Thesis Work, 2002.
Planetary and Space Science 55 (2007) 1190–1196 www.elsevier.com/locate/pss
Miniature ion mass analyzer D. McCanna, , S. Barabasha, H. Nilssona, A. Bhardwajb a
b
Swedish Institute of Space Physics, Kiruna, Sweden Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum, India Accepted 4 July 2006 Available online 12 February 2007
Abstract The Swedish Institute of Space Physics is developing a miniature plasma analyzer for planetary missions (MIPA—Miniature ion precipitation analyzer). MIPA has been accepted to fly on-board both the ESA BepiColombo mission to Mercury (2014) and the Indian Chandrayaan—1 mission to the Moon (2007). The analyzer measures ions in the energy range 10 eV–15 keV and has a sufficient mass resolution to resolve the main groups of ions, namely M=q ¼ 1; 2; 4; 8; 16; 430. Field of view is 9 180 . The instrument consists of the sensor head and a separate electronic board whose total mass is 300 g. The sensor head envelope is roughly 53 85 30 mm3 in volume and is designed for the extreme operation temperature range of 100 to þ125 C. MIPA comprises an electrostatic scanner for angular resolution, a cylindrical electrostatic analyzer for energy discrimination and a time-of-flight (TOF) section for particle velocity measurement. Generic design allows using the instrument on various platforms including nano-satellite and multi-spacecraft missions. This document describes the design of the sensor part including ion optical as well as mechanical aspects. r 2007 Elsevier Ltd. All rights reserved.
1. Introduction The instrument currently being developed at The Swedish Institute of Space Physics (IRF, Kiruna, Sweden) presented here has the working name MIPA (miniature ion precipitation analyzer). In MIPA’s original design it was developed for the European Space Agency BepiColombo (B/C) mission to Mercury, where it is to monitor the precipitating ions (primary) and study sputtered ions from the surface (secondary). MIPA has been proposed and accepted to be included in the SERENA (Search for Exospheric Refilling and Emitted Natural Abundances) package on-board BepiColombo MPO (Mercury Planetary Orbiter). Alongside designing for B/C, MIPA has also been proposed for, and accepted to fly on-board the Indian Chandrayaan—1 mission to the Moon. The working name for the instrument on Chandrayaan—1 is SWIM (Solar WInd Monitor). Small changes to the design (e.g. adding electronic board housing) had to be implemented to meet the constraints, but no sensor-specific properties have been altered, i.e. from a scientific point of view it is the same Corresponding author. Tel.: +46 980 79176; fax: +46 980 79050.
E-mail address: [email protected] (D. McCann). 0032-0633/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2006.11.020
instrument flying on-board both missions. See Table 1 for basic instrument characteristics. Regarding Mercury, placement on the MPO will allow MIPA to study the particles along the magnetics field lines (Sarantos et al., 2001), precipitating upon the surface and inducing sputtering (Lammer and Bauer, 1997). This is a very important component of the ENA (Energetic Neutral Atoms) package of MPO. In order to fully understand the behavior of sputtered ENA’s, one should simultaneously study the source of sputtering. Because of the small size of the Mercury magnetosphere (Kabin et al., 2000), it is only from the MPO platform one can get a good coverage of the plasma populations within the non-tail magnetosphere (Nilsson et al., 2002). Besides the primary objective of monitoring of the solar wind, and supporting the ENA studies, there are several scientific topics related to ions around the Moon that SWIM will investigate, e.g. the lunar-originated ions reflected from the surface, that could give important data about the lunar surface composition (Elphic et al., 1998). Directly connected with the ENA investigations of the possible occurrence of mini-magnetospheres (Harnett and Winglee, 2002), it would also measure ion composition in and around these. This could give information regarding
ARTICLE IN PRESS D. McCann et al. / Planetary and Space Science 55 (2007) 1190–1196 Table 1 Basic instrument characteristics
1191
Table 2 Resource budget summary
Parameter
Value
Resource
Energy range Energy resolution, DE=E Field-of-view Mass resolution Geometric factor
10 eV–15 keV 7% 9 180 H, He, Na/Al, K/Ca, Mn/Fe 0:14 cm2 sr eV
Mass (incl. margins) MIPA Sensor head FEE þ HVPS SWIM Sensor head FEE þ HVPS Electronic board housing
150 g 200 g 100 g
Power consumption MIPA/SWIM
1.5 W
TM MIPA SWIM
TBD 2 kbps (average)
the micro-scale interaction between the anomalies and the solar wind, as well as studying the existence (if any) of ion void regions around the anomalies. Also studying the electromagnetic environment around the terminator should be noted. The terminator is the boundary for photo-electron production, which causes a complicated environment, which in turn could perturb the solar wind (especially the solar wind protons) in this region. Studying the disturbed solar wind in this region could make it possible to map the potential structures around the terminator (Futaana, 2006).
Value
150 g 200 g
2. Mechanical design of the instrument The basic ideas for MIPA were adopted from the proposed PAEX-sensor (Pluto Atmosphere Escape Experiment) (Barabash et al., 2000). The asymmetrical detector geometry of PAEX was replaced with symmetrical plates and the pulse height analysis-based detection of PAEX was replaced by a time-of-flight system (TOF) in MIPA. Table 2 shows mass and power consumption for the instrument. A 3D drawing of the sensor interior can be seen in Fig. 1. The sensor comprises three sub-units. The first is the electrostatic deflector (Section 1 in Fig. 1) consisting of two high voltage plates, which will provide angular resolution. The second part is the electrostatic analyzer (ESA, Section 2 in Fig. 1), which will provide energy (or rather E/q) discrimination. The final sub-unit is the TOF system (Section 3 in Fig. 1), measuring the particle velocity. The sensor will make use of five high voltage supplies (see Table 3). One for each of the two deflector plates (4) and one for the analyzer plate (5). The TOF cell (6, see Section 2.3 for description) will, besides creating a field-free region for optimizing the particle trajectories, provide pre-acceleration as well as bias to the START (7) and STOP (8) surfaces. Finally, both detectors, Ceramic Channel Electron Multipliers (CCEM, 9 and 10) will operate under the same high voltage supply. 2.1. Electrostatic deflector
The deflector consists of two 90 curved high voltage plates with a radius of 19 mm, separated by 2 mm at the aperture. The entrance aperture is entirely covered with a grounded grid (not shown in Fig. 1), which is transparent for ions (transparency efficiency is 90%), so no electric
Fig. 1. 3D view of the sensor head with pointers to the significant subunits and details. The sensor comprise three sub-units; (1) the electrostatic deflector, (2) the electrostatic analyzer and (3) the time-of-flight system. Also seen is the deflector high-voltage plates (4), the analyzer high-voltage plates (5), the time-of-flight cell (6), the START/conversion plate (7), the STOP plate (8), the START and STOP ceramic channel electron multipliers (9, 10) and the UV attenuation block (11).
Table 3 Sensor high voltage supplies Supplies
Interval (kV)
Comment
Upper defl. Lower defl. Analyzer CCEM TOF cell
½3; 5; 0 ½3; 5; 0 ½3; 5; 0 ½0; 5; 0 ½1; 0; 0
Sweeping Sweeping Sweeping Constant Constant
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field will be visible from outside the sensor head. Each of the two deflector plates will scan 90 of the azimuthal acceptance angle. The plate not scanning will be held at ground potential, while scanning will be performed in the interval 0–3.5 kV. In each part of the 180 field-of-view scanning will be performed in four steps for each energy level. Including the zero position (both deflector plates at ground potential) this will provide nine azimuthal sectors. Some problems have been encountered when looking in the extreme angle (90 direction). The angular acceptance distribution becomes asymmetrical (in Section 4 we address this issue in detail). 2.2. Electrostatic analyzer The electrostatic deflector is separated from the electrostatic analyzer by a grounded slit, as is the analyzer from the TOF module. The width of the electrostatic analyzer channel is 2 mm and the height is 20 mm, whereas the entrance- and exit slits have dimensions of 1:5 8:0 mm. This decreases the distorting impact on the fringing fields from the top and bottom edges of the analyzer channel will have on the particle trajectories. The analyzer has a radius of 19 mm and a curvature of 127 , providing focusing of the particle trajectories at the ESA exit (Hughes and Rojansky, 1929). Since, as the case for the deflectors, the ESA is not based on a positive and a negative potential, it is asymmetric. The outer ESA wall is grounded, while the inner wall will be biased with a negative potential to bend the ion trajectories. Assuming a perfect logarithmical distribution for the electric field of the analyzer, it is possible to make a relation between analyzer voltage U A , ion’s kinetic energy E k and charge q for ions with circular orbit which can pass the analyzer (Johnstone, 1972): Ek UA ¼ , q 2 lnðrout =rin Þ
(1)
where the analyzer constant (1=ð2 ln rout =rin Þ) is roughly 5 for MIPA. 2.3. Time-of-flight section Pre-acceleration of 1 keV is used in order to enhance the electron yield from the conversion surface for low-energy ions. In order to suppress electric fields disturbing the particle trajectories in the TOF section, we implemented a structure named the TOF cell. All but for the windows where the two CCEM’s collection plates are placed, it completely closes in the TOF section, creating as field-free region as possible. Fig. 2 shows a zoomed-in view of the TOF cell. The cell is machined out of one piece of graphite, giving uv attenuation as well as low secondary electron emittance. Particles entering the TOF cell will first hit the START surface, giving rise to secondary electron yield, which will be detected by the START CCEM. The ions reflecting off
Fig. 2. Zoomed-in view of the time-of-flight cell. A potential of 1 keV will be applied on the cell, which is machined out of a solid piece of graphite. Graphite will provide both photon flux attenuation, without blackening process, and electron absorption (cmpr e.g. aluminum).
of the START surface will scatter towards the STOP surface, again yielding secondary electrons, which here will be detected by another (STOP) CCEM. Timing of the two detections gives the TOF and since we know the average distance the particle has to travel between the start and stop surface we can determine the velocity. Average TOF distance is 3.1 cm, which corresponds to TOF in the interval a few tens of ns to a couple of ms. CCEMs of type KBL-408 from Dr. Sjuts Optotechnik GmbH were selected for the TOF-system. These will be run typically with an internal potential drop of 2.3 kV. In order to compensate for aging the potential drop can be raised to some 3.3–3.5 kV with an increasing gain. The START and STOP surfaces will be biased to negative high voltage ð1 kVÞ in order to increase the production of secondary electrons as well as guiding the yielded secondary electrons towards the CCEMs. 2.4. UV attenuation Both for the BepiColombo and Chandrayaan—1 mission the instrument aperture will at times be in direct lineof-sight of the sun. In order to suppress ultraviolet radiation at the detector inputs, a photon attenuation structure is added right after the entrance slit of the analyzer. The basic idea of the ‘‘photon trap’’ has been adopted from ASPERA ELS instrument’s top-hat analyzer (Slater, 2000). The trap is a stack of thin copper plates which form a baffle structure having openings of 0.2 mm for photons to fly in. The plates are 0.1 mm thick and are straight cut which will make them act as mirrors for photons hitting the sides of the plates and bounce them back towards the entrance aperture. The length of the plates will follow the ESA’s outer wall radius. Fig. 3 shows a zoomed-in view of the photon trap. The trap as well as all other surfaces in the ion optics of the instrument will be
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Kiruna. The baseline for the TOF electronics can be studied in detail in Svensson and Karlsson, 2002. TOFtiming is performed using A101 charge sensitive preamplifiers (see Fig. 4) for schematic) with a rise time of about 6 ns. A prototype (see Fig. 5) of the TOF electronics has been manufactured and tested for functionality. The HVPS uses analog inputs of 12 V and þ5 V to generate five high voltage outputs. The output high voltages are shown in Table 3. Fig. 6 shows a prototype of a fully functional high-voltage supply for SWIM. 4. Simulations and tests
Fig. 3. Zoomed view of UV attenuation structure. The black body is the analyzer high voltage plate. Photons entering the analyzer channel will either propagate in between the plates and be absorbed or bounce back off of the straight edges. Simulations have indicated as high attenuation as 1010 , but mechanical/integration aspects of the very thin plates makes simulations a bit off-hand.
SimIon 7.0 and a non-commercial particle ray-tracing software (named Trace) have been used for simulations. Fig. 7 shows that for angles up to 60 the angular acceptance is, at least piecewise, symmetric. Again, due to asymmetric geometry the angular acceptance will be
Fig. 4. Simple schematic of the implementation of the A101 charge sensitive amplifiers.
Fig. 6. A picture of a fully functional technological model prototype of the high voltage power supply for SWIM. Board envelope is 120 60 mm.
Anglular distribution 5000
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Fig. 5. A picture of a fully functional technological model prototype of the time-of-flight electronics designed and manufactured at IRF, Kiruna. Board envelope is 120 60 mm.
blackened with Ebanol-C treatment in order to reduce reflections.
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3. Time-of-flight electronics and high voltage power supply Both the TOF electronics and the high voltage power supply (HVPS) are designed and manufactured at IRF,
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Fig. 7. Plot showing the angular distributions for the interval ½70; 60 . The small asymmetric behavior for positive/negative values is due to geometric factors and cannot be avoided.
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different for particles coming from the, by convention, negative angles, since they will not have the same smooth orbital trajectory, as particles incident from positive angles. As seen in the figure, the angular resolution will be a bit more crude for negative acceptance angles. Fig. 8 shows trajectories from START plate to START CCEM for secondary electrons yielded after particles hit the surface. By applying potentials both on the conversion surface (negative) and the detector input (positive) the particles can be guided towards the detectors. Detection efficiency on START plate is X95%. The efficiency of detecting STOP signals is a bit lower (approximately 70%). It is also more dependent on the cell potential. This is due to the obvious reason that the average distance ðdÞ the secondary electrons have to travel is larger than for the electrons yielded at the START surface, but also that the Dd is much larger (i.e. the difference between the shortest TOF for the yielded electrons and the longest). Fig. 9 shows trajectories for secondary electrons yielded at the STOP surface being detected at STOP CCEM. The detection efficiency is quite a bit lower than for electrons yielded at START surface (70%), but as seen in the figure electrons from the entire width of the surface is detected. The detection loss is impinged for particle spread in z (vertical) direction. 4.1. CCEM thermal tests
Fig. 9. Particle trajectories from simulations for second electrons yielded at the STOP surface being detected at STOP CCEM. The detection efficiency is quite a bit lower than for electrons yielded at START surface ð70%Þ, but as seen in the figure electrons from the entire width of the surface is detected. The detection loss is impinged for particle spread in z (vertical) direction.
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Another issue, which is valid for the BepiColombo mission, is that the instrument will be exposed to a significant heating. Thermal calculations have shown that the aperture can be heated as high as almost 300 C. This will not have implications for the electronics, as the electronics board housing will be inside the S/C wall, but it means that the CCEMs will operate under high
Fig. 8. START CCEM secondary electron collection from ray-tracing simulations. Detection efficiency of electrons from the START surface quite is high ðX95%Þ.
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Fig. 10. Plot showing the normalized degradation in gain as a function of temperature of the CCEM’s. Full gain corresponds to 108 . This test was just instantaneous gain fall due to temperature increases, not long time duration.
temperature decreasing the signal-to-noise ratio. The temperature inside the instrument will not be as high as at the aperture, but we can anticipate a temperature of some 125 C. Thermal-vacuum tests have been made to investigate the effect of high temperature operations of the CCEMs. Fig. 10 shows the normalized gain drop as a function of temperature. The CCEM bias is held constant throughout the test. Studies of how the noise would decrease with time (accumulated counts) are very important, since, at high temperatures, the noise could be a significant source of error. Fig. 11 shows how the gain (top) and noise (bottom) drop with accumulated counts on the CCEM. The temperature was held constant at 125 C throughout the test period and the CCEM bias was held constant at 2.3 keV. The gain stabilizes at about an order of magnitude attenuation, which still gives an acceptable gain. The noise stabilizes at some 30 counts/s, which gives a correlated noise of about 1 counts/s. These noise levels are fully acceptable.
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SWIM TM Time-Of-Flight Spectra 3keV H2O+ 10
0.80 0.60 0.40 0.20 0.00 8.64E+07
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Fig. 11. Top: the normalized gain drop due to degradation of the CCEM at high temperature. The gain stabilizes at about an order of magnitude attenuation, which still gives an acceptable gain. Bottom: simultaneous noise drop with accumulated counts. Noise of the order of tens of Hz will not be a source of error. The temperature was held constant at 125 C and the potential drop at nominal 2.3 kV.
SWIM TM Time-Of-Flight Spectra 1.6 +
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TOF [ns] þ Fig. 12. Two spectra overlaid: 2.1 keV Hþ 2 and 2.1 keV H2 0 . The heavy projectiles (water) show a recoil peak, whereas in the Hþ 2 case, no separated peak is visible due to closer mass match. Positions of the individual peaks are as expected. The dotted line represents the fastest possible flight time for recoil Hþ .
5. Calibrations Initial calibrations have begun for a technological model of the instrument. Calibrations is performed at IRF’s
H2O+, 3keV, Vtof = 0V H2O+, 3keV, Vtof = 500V peak: 179ns, gauss:19ns, exp: 69ns peak: 171ns, gauss:17ns, exp: 66ns
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Fig. 13. The effect of a changed TOF cell potential as well as the recoil and nominal particles separated. The max spread in TOF introduced by the extended start surface is shown as rectangular function. The translation of the peak for a TOF cell potential (pre-acceleration) a0 is as expected.
vacuum chamber facilities in Kiruna, Sweden, and some first calibration results will follow here. Fig. 12 shows TOF distributions for two particle species, þ Hþ 2 and H2 O , at 2.1 keV. The curve for the heavy particles clearly shows a second peak in the spectrum. This peak originates from proton recoils generated the START surface. The heavy particles simply knock protons from the START surface that will reach the STOP surface before the primary heavy particle, and hence the TOF for that signal will be shorter, resulting in the second peak. This problem is partly related to the environment in the vacuum chamber and will not be as significant in space, but it will most likely not disappear entirely. The light particles ðHþ 2Þ shows a narrow single peak curve, which is as expected. Fig. 13 shows the pre-acceleration effect from increased TOF cell potential (the right-most curve is under no preacceleration, while the left-most is pre-accelerated 500 V). For the energy shown (3 keV) it will not have a significant narrowing of the peaks as for lower energies, but one can see the shift in TOFs for the different conditions. Recoil protons are clearly visible in this plot as well. Thorough investigations into the characteristics of MIPA/SWIM will be performed over the next few months, where we will look at the entire energy spectrum for the þ þ þ þ masses Hþ , Hþ 2 , H3 , N2 , H2 O and Ar . Acknowledgments Martin Wieser has contributed very much with his experience and help during the calibrations of SWIM TM. We would also like to acknowledge Peter Wurz and his team (University of Bern) for valuable assistance and help regarding certain hardware issues.
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Kabin, K., et al., 2000. Interaction of Mercury with the solar wind. Icarus 143, 397–406. Lammer, H., Bauer, S.J., 1997. Mercury’s exosphere: origin of surface sputtering and implications. Planet. Space Sci. 45 (1), 73–79. Nilsson, H., Holmstro¨m, M., Barabash, S., 2002. Feasability of a charged particle detector on the Mercury Planetary Orbiter. Internal report, Swedish Institute of Space Physics. Sarantos, H., Reiff, P.H., Hill, W.T., Killen, R.M., Urqhart, A.L., 2001. A Bx -interconnected magnetosphere model for Mercury. Planet. Space Sci. 49 (14–15), 1629–1635. Slater, D.C., 2000. ASPERA ELS UV supression study. Final report, Southwest Research Institute. Svensson, J., Karlsson, S., 2002. Time-of-flight electronics based on TDCGP1. Thesis Work, 2002.